The Dehavilland DHC-2 is a common transport aircraft. This plane has a heating system for the cabin that uses the warm exhaust to heat air for the cabin. The heat exchanger unit is built into the exhaust system. Heat is fed into the cabin by a flexible duct. The heat is controlled from the cabin by a damper at the duct connection to the exhaust system. When heat is not needed, the damper is set to allow the warm air to vent to the atmosphere. When heat is needed, the damper is moved, by a cable, to permit the warm air to flow into the cabin. The damper is typically mounted in a small heat box at the rear end of the exhaust tailpipe. The damper is made from a flat rectangular plate type "321" stainless steel that has a small shaft sleeve welded across the top. This sleeve pivots on a shaft that moves the damper to the appropriate position. After continued use, the vibration and heat tend to warp the damper and the housing. Often the damper is so badly damaged that the heat box must be rebuilt or replaced. If the airplane is used frequently, this procedure can be required several times a year.